How to Specify Sprue Bushing SR (Spherical Radius) for Your Machine

Understanding SR

The SR (Spherical Radius) defines the curvature of the concave pocket on the sprue bushing face where the machine nozzle tip presses during injection. The nozzle has a convex spherical tip; the bushing has a matching concave sphere. When the two radii are properly matched, the nozzle seats firmly in the bushing, creating a seal that prevents resin leakage.

The relationship is simple:

• Bushing SR must be slightly larger than nozzle SR
• This ensures the nozzle tip contacts the bushing across a broad area around the sprue port, not just at the edge

Common Nozzle SR Values by Machine Brand

These are typical values. Always verify against the actual nozzle specification for your specific machine model. According to Euromap standards, nozzle SR is specified in the machine technical data sheet under "Nozzle specifications" or "Nozzle geometry."

How to Measure Nozzle SR

If the machine documentation is unavailable, measure the nozzle SR directly:

• SR gauge method — Use a set of SR gauges (concave gauge blocks with known radii). Place each gauge on the nozzle tip and find the one that makes full contact across the sphere.
• Clay impression method — Press modeling clay against the nozzle tip to create an impression, then measure the impression radius using calipers and the chord/sagitta formula: R = (c²/8s) + s/2, where c is chord width and s is sagitta depth.
• Contact your machine supplier — The fastest and most reliable method. Provide the machine model number and serial number.

What Happens When SR Is Wrong

According to JIS B 5111 standards, the sprue bushing SR specification must be documented on the mold assembly drawing and verified during mold commissioning.